The Toarcian OAE or T-OAE occurred during the Early Jurassic Period approximately (~183 million years ago). The leading hypothesis for the cause of this event are massive volcanic eruptions associated with the emplacement of the Karro-Ferrar Large Igneous Provence. The gases emitted by these eruptions resulted in a transient interval of global warming and series of severe environmental changes that resulted in an extinction event. Our studies have sought to identify and quantify the environmental changes that occurred during this event. One of these environmental perturbations was the development or expansion of anoxia in parts of the oceans at time – hence the term oceanic anoxic event.

The first paper, published in Earth and Planetary Science Letters this past February, concerns the record of the carbon cycle during the T-OAE.

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This paper presents the carbon isotope stratigraphy from one of our study sites in Western North America (East Tributary of Bighorn Creek, Alberta, Canada). This study showed that the changes or excursions in the carbon isotope stratigraphy found elsewhere1,2,3,4 can be found in the North American record. This includes the large-scale excursion (see figure below), but also smaller-scale features in this record. This observation is important because this indicates that these excursions represent changes in the global carbon cycle. We also argue for an important role of the terrestrial carbon cycle in the overall carbon cycle and climate feedbacks during the event.

Carbon isotope stratigraphy and ammonite biostratigraphy from the East Tributary study site. Note the large carbon isotope excursion (CIE) that occurs in the T-OAE interval.

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Our second paper, published in Scientific Reports in mid-July, involves reconstructing the changes in chemical weathering over the T-OAE using osmium isotopes. Chemical weathering is an important negative feedback to rising carbon dioxide levels in the atmosphere; these chemical reactions remove carbon dioxide from the atmosphere. We found an osmium isotope excursion – an increase in the ratio of the osmium isotope 187 (187Os) over 188 (188Os) (see plot below) – during the interval that contains the T-OAE.

Carbon and osmium isotope stratigraphy from the East Tributary study site. Note the osmium isotope excursion that occurs at the same level as the T-OAE carbon isotope excursion.

This excursion indicates a transient increase in the weathering of continental rocks during the event; continental rocks are in enriched in 187Os relative 188Os as compared to other sources of osmium to the ocean. While similar signals have been previously found in locations in Europe6,7, it had yet to be identified elsewhere. Again, as for the carbon isotope record, this observation is critical to identify whether the excursion represents a global change in the osmium cycle. We also went a step further and used a numerical model of the osmium cycle to derive estimates of the change in chemical weathering during the event: our model suggests up to 3-fold increase in the weathering of continental material during the T-OAE.

Stay tuned for more work from our group on the Toarcian Oceanic Anoxic Event. We have several on going studies of this fascinating time interval in the history of the Earth.